This invention generally relates to liquid crystal compositions, and more specifically to polymeric materials in a liquid crystalline state.
Liquid Crystal Polymers
It has generally been thought that in order for normally flexible polymers to display liquid crystalline characteristics, rod-like or disk-like elements, i.e. mesogens, must be incorporated into their chains. The placement of these mesogens typically controls the type of liquid crystalline (xe2x80x9cLCxe2x80x9d) polymer formed. Polymer liquid crystals (PLCs) generally can be divided into two types: main-chain PLCs and side-chain PLCs. Main-chain polymer liquid crystals are formed when the mesogens are themselves part of the main chain of a polymer. Conversely, side chain liquid crystal polymers are formed when the mesogens are connected as side chains to the polymer by a flexible bridge, or spacer.
Thermoplastic polymers combined with mesogens have been extensively studied because the ordered fluid phases of liquid crystals offer unique properties useful, for example, as precursors to high performance polymeric films, fibers, and injection molded articles. For example, U.S. Pat. No. 4,668,760 to Boudreaux Jr., et al. describes a process that includes synthesizing a liquid crystal polyester, devolatilizing the liquid crystal polyester, and then shaping the devolatilized polyester into an article of manufacture, such as fibers useful in tire cords. These polymers have mostly been aromatic copolyesters, since many polymers, including some aromatic homopolyesters, have melting points too high to form thermotropic mesophases without decomposition.
Temperature and Pressure Induced Phase Changes
Elevated pressure is known to reversibly induce the formation of a liquid crystalline state in mesogenic polymers. For example, Hsiao, et al., Macromolecules 21:543-45 (1988) discloses a process of heating a sample of HIQ-20 (a copolyester) above the clearing temperature (342xc2x0 C.), applying a pressure of up to 6000 bar (0.6 GPa) to the sample, reducing the temperature to the mesophase temperature, maintaining the temperature for 1 hr., and then cooling the sample to room temperature at a rate of 3xc2x0 C./min. It was found that cooling the mesophase into the solid state under moderate pressure yielded a morphology that differed from that in the solid cooled at ambient pressure. The study was limited to the mesogenic polymer, HIQ-20.
Maeda, et al., Macromolecules 28:1661-67 (1995) describes a study on the thermotropic polymer (4,4xe2x80x2-dihydroxybiphenyl) tetradecanedioic acid polyester (PB-12), which is known to exhibit liquid crystalline properties. The phase transition of PB-12 under hydrostatic pressures up to 300 MPa was observed. The typical phase transition of crystal (K)-smectic H (SH)-isotropic melt (I) was observed under hydrostatic pressures up to 90-100 MPa and elevated temperatures. A new smectic phase was formed irreversibly from the usual SH phase by increasing pressure on a quasi-isothermal process. After heating above the clearing temperature, the sample was supercooled at high pressures, and the glassy SB phase was found coexistent with the normal crystals at room temperature under atmospheric pressure. Other thermotropic polyesters studied include two homopolymers and the corresponding copolymer based on 4,4xe2x80x2-biphenyldiol as the mesogen and aliphatic dibasic acids containing 7 and 8 methylene groups as flexible spacers (Maeda, et al., Makromol. Chem, 194:3123-34 (1993)).
Phase transitions as a function of temperature and pressure have been studied on other select polymers. Rastogi, et al., Nature 353 (1991) examined poly(4-methyl-pentene-1), which is crystalline under ambient conditions, and which was found to become reversibly amorphous on increasing pressure in two widely separate temperature regimes (approximately 20xc2x0 C. and 200xc2x0 C.). The transformation occurred via liquid-crystal and amorphous phases as pressure or temperature was varied. The liquid crystalline state was not retained when returned to ambient conditions. Polytetrafluoroethylene and polyethylene also have been examined for structure of high pressure phases, as described in Tetuo Takemura, xe2x80x9cStructure and physical properties of high polymers under high pressurexe2x80x9d (Reprint of a paper read at November 1978 Meeting of Polymer Science in Japan) and Plate and Shibaev, xe2x80x9cComb-Shaped Polymers and Liquid Crystalsxe2x80x9d (Cowie, ed.) pp.207-09 (Plenum Press, New York 1987). The references do not indicate retention of a liquid crystalline state in these polymers at ambient temperature after applying pressure.
Efforts to use liquid crystalline materials in controlled release systems are described in U.S. Pat. No. 5,753,259 to Engstrom, et al. These non-polymeric systems include a cubic liquid crystalline phase and purportedly provide a highly reproducible controlled drug release system, in contrast to solutions involving polymers.
PCT WO 98/47487 discloses a drug delivery composition that includes an active substance (e.g., drug) and a fatty acid ester substance capable of forming a liquid crystalline phase in the presence of a liquid medium. In these compositions, which can be mixed with polycarbophiles, the lipid forms a liquid crystalline state, but the polymer itself does not. Furthermore, the requirement of a liquid medium, particularly water, significantly limits the forms and uses of the compositions.
It is therefore an object of this invention to provide non-mesogenic polymers that exhibit liquid crystalline properties at ambient temperatures.
It is a further object of this invention to provide methods for inducing a liquid crystalline state in any thermoplastic polymer, preferably in the substantial absence of water.
It is a further object of this invention to provide methods for inducing a liquid crystalline state in cross-linked polymers.
It is another object of the present invention to provide a liquid crystalline polymer that retains its liquid crystalline state for an extended period of time, such as several hours or years.
It is another object of this invention to provide non-mesogenic polymer systems for the controlled release of a variety of molecules, including therapeutic and diagnostic agents, as well as cosmetics and fragrances.
It is still a further object of this invention to provide methods for reducing the permeability of various polymers to molecules, such as gases or fragrances, by inducing liquid crystalline properties in the polymers.
It is another object of the present invention to provide compositions including polymers such as high- and/or low-density polyethylene having improved physical or mechanical properties which are useful in various applications.
It is also an object of the present invention to provide methods and articles for displaying information using polymers that exhibit liquid crystalline properties at ambient temperatures.
It is another object of the present invention to provide a method of inducing unique liquid crystalline states in mesogenic polymers.
Methods are provided for inducing a polymer, which can be non-mesogenic or mesogenic, to exhibit liquid crystalline properties. The method includes the steps of (a) heating the polymer from an initial temperature below its glass transition temperature (Tg) to a temperature greater than its Tg and below its melting temperature (Tm); (b) exposing the polymer to a pressure greater than about 28 MPa (2 metric tons/in2), preferably between about 28 and 140 MPa (2 and 10 metric tons/in2), typically for between about 30 seconds and 5 minutes, preferably for at least about one minute, while maintaining the temperature greater than its Tg; and (c) cooling the polymer below the Tg while maintaining the elevated pressure, typically for between about 30 seconds and 5 minutes. Unlike many prior art transition processes which are reversible at ambient conditions, this process produces a liquid crystalline state, or another new state with similar characteristics, that can be maintained for years at ambient conditions, even after removing the pressure.
Methods for identifying polymers having liquid crystals (xe2x80x9cLCxe2x80x9d) or non-LC ordered phases include those known in the art, such as optical pattern or texture observations with a polarizing microscope, differential scanning calorimetry, miscibility or density comparisons, molecular orientations by either supporting surface treatments or external fields, and classical x-ray and x-ray diffraction techniques.
Polymer can be bioerodible or non-bioerodible. Representative non-mesogenic, bioerodible polymers include polylactic acid, polylactide-co-glycolide, polycaprolactones, polyvaleric acid, polyorthoesters, polysaccharides, polypeptides, and certain polyesters. Representative mesogenic, bioerodible polymers include some polyanhydrides and polybutylene terephthalate. Preferred non-mesogenic, non-erodible polymers include polyethylene, polypropylene, polystyrene, and polytherephthalic acid. The polymer can be water-soluble or water-insoluble.
The liquid crystalline polymers described herein can be used in the controlled release or retention of substances encapsulated in the LC polymers. The polymer can be in a variety of forms including films, film laminants, and microparticles. In a preferred embodiment, the LC polymers are used to encapsulate therapeutic, diagnostic, or prophylactic agents for use in medical or pharmaceutical applications. Other substances which can be encapsulated include scents such as perfumes, flavoring or coloring agents, sunscreen, and pesticides.
The methods of inducing liquid crystalline properties in polymer also can be used to improve the permeability of polymers in numerous applications, such as packaging, particularly food and pharmaceutical packaging. The methods similarly can be used to enhance the structural performance of polymeric devices, such prosthetics made of polyethylenes.